1. Field of Invention
The present invention relates to ossicular prostheses, more particularly to an improved stapedial prosthesis.
2. State of the Art
Medical Background: Stapes Diseases
Referring to prior art FIGS. 1 and 2, in a normal ear, sound energy, which consists of vibrations of air molecules, is directed by the auricle, or outer ear, through the ear canal to the tympanic membrane. Movements of the tympanic membrane are transferred to the ossicles, or the ossicular chain of the bones of the middle ear: malleus, incus and stapes. These movements eventually reach the inner ear labyrinth fluids via the stapes, which rests in a small groove, commonly called the oval window. The oval window is in intimate contact with the inner ear fluids. The movement of the inner ear fluids then stimulates the fine sensory organs of the inner ear, which in turn stimulate the auditory nerve, finally transferring the original sound energy to the brain.
Certain pathologies within the middle ear ossicular chain interrupt the transmission of sound energy and cause hearing loss. When this occurs, re-construction of the ossicular chain, using man-made prostheses, is often required. Three general conditions of discontinuity exist in which a prosthesis is appropriate. When the malleus, incus and stapes are absent, a Total Ossicular Replacement Prosthesis (or TORP) is used. When the stapes is absent but the malleus and incus are present, a Partial Ossicular Replacement Prosthesis (or PORP) is used. When the stapes footplate is fixed due to a condition referred to as otosclerosis, a stapes prosthesis is used.
Otosclerosis is one of the most common causes of progressive hearing loss in which there is an abnormal growth of bone in the ear. When otosclerosis is present there is an abnormal, microscopic growth of bone in the walls of the middle ear. This abnormal growth impedes the conduction of sound energy from the tympanic membrane to the inner ear. In particular, otosclerosis affects the stapes bone by causing it to become frozen in place or “fixed.” Normally the stapes vibrates freely to allow the transmission of sound energy into the inner ear. When it becomes fixed to the surrounding bone, it prevents sound waves from reaching the inner ear fluids, and thus hearing is impaired.
When the amount of otosclerosis at this location is significant, as determined by careful hearing tests, surgery has been found to be the most effective method of improving hearing loss caused by this condition. This surgery is termed a stapedectomy and serves to restore continuity between the incus and inner ear fluids. A stapedectomy is sometimes performed in patients who have a congenital abnormality of the stapes or have sustained a fracture of the stapes from traumatic incident. However, the most common indication for a stapedectomy is otosclerosis.
Medical Background: Stapedectomy Surgery Technique
Surgical treatment for otosclerosis has been available for about 45 years. The first operation for this disease was the fenestration procedure, which required mastoid surgery and an artificial opening in another part of the inner ear. The attention of surgeons became focused on the diseased stapes itself and the stapes mobilization procedure was developed. With the improvement in surgical techniques, the treatment of choice became stapedectomy. This procedure was first performed in 1956 and has remained the mainstay of treatment for otosclerosis.
The objectives of stapedectomy are: (1) to open the oval window for sound entry into the inner ear labyrinth; (2) to reconstruct a conductive bridge between the incus and the labyrinth; and (3) to accomplish these goals as efficiently and physiologically compatible as possible for long-term hearing without complication. To accomplish these objectives the stapedectomy is performed through an incision in the ear canal under local or general anesthesia. A flap consisting of canal skin and tympanic membrane is elevated and the posterior superior bony external auditory canal wall is removed, usually by a drill, to expose the malleus, incus, stapes and chorda tympani (facial nerve). The ossicles are inspected and palpated to establish the precise diagnosis of the conductive hearing loss; that is, the fixation of the stapes and mobility of the malleus and incus are confirmed. The distance between the undersurface of the incus and the stapes footplate is then measured to determine the prosthesis length.
With care taken to preserve the chorda tympani, the synovial joint between the lenticular process of the incus and the head of the stapes is separated (incudostapedial joint). The stapes tendon and one crus (leg) of the stapes is then severed. The arch of the stapes may then be removed by fracturing the other crus while allowing the stapes footplate to remain in the oval window. An opening is created in the footplate to allow entrance for a stapedial prosthesis. In some cases, the footplate is removed and a vein is grafted to the internal wall of the tympanum to cover the opening and to support the stapes prosthesis. After the opening is made in the footplate, or tissue is placed over the opening made to the inner ear after removing the footplate, one end of a stapedial prosthesis of proper length is posted in the opening while the other end is attached to the incus. The incus is gently palpated to observe the motion of the stapedial prosthesis. A piece of fat or tissue is applied (obtained, as one example, from a small incision behind the ear lobe) in order to seal any hole in the oval window. Finally, the eardrum is folded back into its normal position.
Medical Background: Prior Art Prostheses
A critical part of the stapedectomy procedure is attaching the prosthesis around the lenticular process of the incus due to its miniature size, typically about 3.5 mm to 6 mm long and 0.4 mm to 0.8 mm diameter, and its delicate nature. There have been several devices proposed for the stapedial prosthesis. One class includes those that use a crimping technique. For instance, in U.S. Pat. No. 5,370,689 to Causse, one end of the prosthesis, fabricated of PTFE, is posted in an opening drilled in the exposed footplate, and a split eyelet at the other end must be crimped around the incus. In U.S. Pat. No. 3,711,869 to Shea Jr. one end of the prosthesis is placed on a vein graft invaginated into the oval window, and a split eyelet at the other end must be forced open by elastic deformation to fit onto the incus. Elastic recovery capacity of the eyelet causes it to restore to its original form in about 20 minutes and grip the incus firmly. U.S. Pat. No. 3,838,468 to Armstrong discloses a stapedial prosthesis for use in cases where the footplate is also removed. A piston is fixed at one end to a vein graft for covering the oval window. A wire of stainless steel, platinum, gold, or like biocompatible material shaped like a shepherd's crook extending from the other end, is crimped about the lenticular process of the incus.
Another type of prosthesis in use is the “bucket-handle prosthesis,” which in comparison to the traditional crimped stapes prosthesis discussed above, is reportedly easier to insert (as discussed by Rothbaum, et. al in “Task performance in stapedotomy: Comparison between surgeons of different experience levels,” Otolaryngol Head Neck Surg. 128:71–7 (2003)). Referring to prior art FIGS. 3 and 4, U.S. Pat. No. 3,196,462 to Robinson discloses one type of bucket-handle device 10, also termed the bucket-and-bail device, which includes a bucket 12 (also termed a well or a socket), a wire bail 14 (handle) at one end for receiving and securing a portion of the disarticulated lenticular process 16 of the incus 18, and a cylindrical shaft portion 20 (or stem or rod) at the other end for engaging the oval window 22, or tissue 24 (e.g., a vein graft or fat tissue) that is placed over the opening made to the inner ear after removing the footplate. The bail 14 is oriented by the surgeon so that its axis of pivotal rotation generally is horizontal. With the prosthesis 10 in that configuration, the incus lenticular process 16 is positioned in the bucket 12 and the bail 14 is rotated in an upward arc about a fulcrum point on the bucket, past the horizontal, until the bail contacts the long process of the incus 18. In this particular device, only the frictional engagement of the bail 14 against the incus 18 holds the bail in place. If the bail should rotate downwardly, out of range of contact with the incus, the prosthesis may become dislodged and extrusion may result. In contrast, with a crimpable wire-crook prosthesis, a short term risk is that the prosthesis will fracture the incus. Additionally, a loose fitting crimpable prosthesis may erode or wear away the incus from irregular loose vibration. Thus, the challenge of stapedial prostheses, whether a bucket-and-bail type or a crimpable type is to secure the prosthesis sufficiently well to the incus to avoid its failing out of position while being careful not to have a connection to the incus that allows the incus to erode or to fracture.
U.S. Pat. No. 4,292,693 to Shea et. al discloses one means for overcoming the bail securement problem. Referring to prior art FIGS. 5 and 6, the stapedial prosthesis 40 of Shea et al. provides a pair of cam surfaces 42 oppositely mounted on the bucket portion 44 which permits overcenter pivotal movement of the bail 46 in one direction and thereafter prevents overcenter pivotal movement of the bail in the opposite direction. Each of the cam surfaces 42 tapers outwardly from the surface of the bucket portion 44 in the direction the overcenter pivotal movement of the bail 46 is permitted, and terminates in an inwardly extending shoulder portion 48 which engages the bail 46 to prevent the overcenter pivotal movement thereof in the opposite direction.
Currently, these bail handles are constructed from stainless steel. Current attachment methods of the bail handle to the bucket in bail-and-bucket device require that the bucket handle protrude into the bucket of the prosthesis, thus disturbing the incus process as it lies in the bucket, which further creates problems in measuring the prosthesis bucket for a correct fit. During the manufacturing process, a through-hole 50 is first made completely through the bucket body 44. The bail 46 is then formed into a generally elongated D-shaped configuration with ends turned inwardly at right angles. The bail 46 is preferably formed of a single strand of stainless steel wire with its ends 52, 54 joined in abutting relationship as by a weld 56, twisting or crimping. The stainless steel material of the bail 46 is such that one end may be deformed and inserted completely through the hole 50 so that the weld 56 may be made. Thereafter, the weld 56 is pulled into the through-hole 50 until it is approximately centered and the deformed portion resumes its original shape. This securement method results in a variability of size and maneuverability in the bail handle, which further complicates the surgical insertion of the device.
It is readily apparent that great care and skill are required to secure these and similar prostheses to the lenticular process of the incus. The minute size of the prostheses also makes them extremely difficult to manipulate into proper position for tightening around the incus, even with state-of-the-art microsurgical instrumentation. This is made more difficult as during the stapedectomy surgery, the view at the prosthesis insertion site is restricted.
As mentioned above, measurement and selection of the prosthesis, which is affected by current manufacturing processes, is critical for proper functionality. Once in place, if the prosthesis is not tightened sufficiently about the incus, fluctuating hearing loss, dizziness, or extrusion of the prosthesis may occur. If it is too tight, necrosis of the incus may occur. In either case, the securement method in itself may cause trauma to the delicate middle ear structures, including fracture or subluxation (dislocation) of the incus.
Therefore, from the above, it can be appreciated that there is a need for an improved bucket-handle prosthesis, particularly with respect to manufacturing repeatability of the bail handle and easier and more proficient attachment to the incus process.